If that's all you want, you can absolutely use git as a monorepo, and you can check in vendored dependencies just as easily.

The main downside (and I admit it can be a big one) to a git monorepo (vs. svn) is that you can't check out a subtree all that easily.

(I do agree with the parent that svn externals is a lot more seamless than git submodules, though.)